This invention relates to electrostatically coatable compositions based on thermosetting polymers and, more particularly, relates to such compositions based on sheet molding compound or bulk molding compound.
It is known to prepare coated articles by electrostatic painting methods. In such methods, a paint or coating is charged or ionized and sprayed on a grounded, conductive article, and the electrostatic attraction between the paint or coating and the grounded article results in a more efficient painting process with less wasted paint material, and thicker and more consistent paint coverage, particularly when the article has a complex shape. When articles fabricated from metals are painted, the metal, which is inherently conductive, is easily grounded and efficiently painted. In recent years, there has been an emphasis on the use of polymeric materials in the manufacture of articles, particularly in applications requiring reductions in weight and improved corrosion resistance, such as automotive applications. However, polymers typically used in such processes are insufficiently conductive to efficiently obtain satisfactory paint thickness and coverage when the article is electrostatically painted.
Methods are known for the incorporation of conductive fillers into polymers in order to improve their conductivity for use in electrostatic coating applications. However, the conductivity of articles made therefrom, as well as the physical, and/or surface appearance properties of the coated articles, may be less than desirable for certain applications. The use of conductive primer compositions to prime the article in order to increase its conductivity is also known. However, depending on the particular primer employed, the cured primer may have adhesion, surface smoothness, hydrolytic stability, and durability characteristics, which are less than desirable for a particular application. In addition, such primers typically contain volatile organic solvents, the emission of which during the priming process may be undesirable.
U.S. Pat. No. 5,490,893 illustrates a method for making a laminate of a thermoformable conductive material and an article of sheet molding compound (SMC) to provide an SMC-based article having good surface conductivity for use in electrostatic coating applications. However, the use of such laminates, or the use of conductive primers, represents extra steps in the forming of the article, the addition of which is less than desirable in a commercial process. European Patent Application No. 623782 describes a method for making vehicle headlight reflectors by injection-molding a bulk-molding compound that contains conductive carbon black. However, bulk-molding compound typically contains a relatively lower proportion of glass fibers, and fibers having a shorter length (which may result in relatively lower physical properties), than SMC, which makes its use less than desirable for certain applications.
SMC is a moldable thermosetting material that is used to prepare structural parts for a variety of applications, including automotive. The material is typically prepared in a continuous process by depositing glass fibers between two or more sheets of a high-viscosity composition comprised of thermosetting polyester resin, calcium carbonate, and alkaline earth metal oxide- or alkali-metal hydroxide- based thickeners between two carrier films of polyethylene. This "sandwich" type of composite is then rolled up and allowed to "mature" and thicken for a period of time to increase the viscosity and handleability of cut portions of the material. The matured rolls of material are then cut into desired shapes for molding, the carrier film is removed, and the SMC is then compression molded into the desired three-dimensional shape of a part having good surface characteristics and physical properties.
Adjusting the formulation of SMC requires a balancing of the desired characteristics of handleability of the uncured SMC material, relatively low viscosity of the resin composition used to make the material (since the resin composition is pumped using conventional pumping equipment instead of high-shear extruders), low cost (achieved by minimizing the amount of resin and maximizing the amount of calcium carbonate), as well as the surface gloss and physical characteristics of the final part. The resin demand characteristics of the components to be added to the formulation and the cost of the proportionate amount of resin needed to achieve the desired low viscosity are several factors that must be taken into account. Carbon black has a high resin demand, relative to glass fiber, calcium carbonate, and the thickening agents typically employed in the preparation of SMC.